In oil and gas well completion operations, frac and/or bridge plugs are necessary for zonal isolation and multi-zone hydraulic fracturing processes. The advantages of frac and bridge plugs made primarily from composite materials is well established since these products significantly reduce drill-out (removal) time compared to drill out time for predominantly metal frac and bridge plugs. As drilling for oil and gas extends deeper and/or fracking pressures increase, composite frac and bridge plugs are subject to higher pressures and operating temperatures. With higher pressures and operating temperatures, increased stresses can be expected on frac and bridge plug products resulting in a corresponding need to engineer higher grade components, which can also mean increased in manufacturing costs. But as use of composite frac and bridge plug products increases, end users expect a corresponding decrease in costs.
Typical frac and bridge plugs can have upper and lower sleeves that are mounted or connected to a tubular central mandrel. For different manufacturers, the upper and lower sleeves may have other names or references, such as being called upper and lower support members, retaining bands, and loading rings. In general, the upper and lower sleeves retain the plug packing elements and slip elements in between the two sleeves to both lock the plug to the well pipe casing and seal the casing so hydraulic fracking pressurization requiring well isolation can occur. For a bridge plug, the mandrel is blocked from flow in either direction through the bore of the mandrel. In practice, this can be accomplished by providing a mandrel with a solid core or by closing the bore of the mandrel with an internal plug. For a frac plug, a closing ball is typically dropped to block the bore of the mandrel to isolate higher pressure above the frac plug from a lower pressure below. As readily understood, a cost effective but reliably strong upper and lower sleeve connection scheme for composite frac and bridge plugs is highly desirable.
When a frac or bridge plug is “set” in the well pipe casing, slips engage the casing and wedge the plug in place. Additionally, the rubber packing element is compressed axially to expand radially outwardly of the mandrel to seal against the casing inner wall. When the frac ball or closing ball is dropped for a frac plug, or when a bridge plug is set and the well is pressurized, the composite plug must withstand injection pressures as high as 8,000 psi, 10,000 psi, or even 15,000 psi depending on the well project.
The connection of the composite upper sleeve to the mandrel is what prevents the closing ball and the mandrel that it seats against from slipping past the packing element and slip components. The upper sleeve is the primary restraint that prevents the frac plug mandrel from being pushed or blown down the well casing. In the case of a bridge plug, both the upper sleeve and the lower sleeve experience a high force as pressure is reversed from above and below the plug. Consequently, a high strength connection of the sleeve and the mandrel is required for frac and bridge plugs.
A high strength connection is typically achieved by multiple mechanical shear pins or a combination of adhesive bonding and multiple mechanical shear pins between the sleeve and the mandrel. Mechanical shear pins are generally considered the most reliable way to achieve a high strength shear connection, although pins are considered labor intensive and costly. To form a pinned connection, multiple holes must be formed through the sleeve and the mandrel and then high strength composite or metal pins pressed in or glued in place in the drilled holes. Adhesive bonding with no pins can, in certain products, achieve the necessary strength for the application but has the potential of high variability part to part and the achieved strength is dependent on the adhesive bonding process.
Additionally, experience has shown that lengthening the sleeve and the mandrel overlap to provide added adhesive bond length between the two does not yield a linear increase in strength. Thus, a combination of adhesive bonding and mechanical shear pins is generally considered the more reliable sleeve to mandrel connection to date. Undesirably, the process of drilling the holes and installing the pins is time consuming and comes with a cost that is less desirous for users and consumers. Additionally, the holes drilled for pins are potential leak sites and points for hydrothermal degradation of the composite downhole plug.